Distinct roles for C3a and C5a in complement-induced tubulointerstitial injury. Kidney Int. 2011;80:524-534. [PMID: 21677637 DOI: 10.1038/ki.2011.158]

Renal Epithelial Complement C3 Expression Affects Kidney Fibrosis Progression

Kidney fibrosis is a hallmark of chronic kidney diseases. Evidence shows that genetic variability and complement component 3 (C3) might influence tubulointerstitial fibrosis. Still, the role of renal C3 production in the epithelial-to-mesenchymal transition (EMT) and genetically determined fibrosis progression remains undiscovered. The kidneys of fibrosis-resistant C57Bl/6J (B6) and fibrosis-prone CBA/J (CBA) and BALB/cJ (BalbC) mice (n = 4-8/group) were subjected to unilateral ureteral obstruction (UUO) and analyzed after 1, 3, and 7 days, along with human focal glomerular sclerotic (FSGS) and healthy kidneys. Mouse primary tubular epithelial cells (PTECs) were investigated after 24 h of treatment with transforming growth factor β (TGFβ) or complement anaphylatoxin 3a (C3a) agonist (n = 4/group). UUO resulted in delayed kidney injury in fibrosis-resistant B6 mice, but very early renal C3 messenger RNA (mRNA) induction in fibrosis-prone CBA and BalbC mice, along with collagen I (Col1a1) and collagen III (Col3a1). CBA depicted the fastest fibrosis progression with the highest C3, lipocalin-2 (Lcn2), Tgfb1, and chemokine (C-C motif) ligand 2 (Ccl2) expression. Human FSGS kidneys depicted C3 mRNA over-expression and strong tubular C3 immunostaining. In PTECs, C3a agonist treatment induced pro-fibrotic early growth response protein 1 (EGR1) expression and the EMT, independent of TGFβ signaling. We conclude that de novo renal tubular C3 synthesis is associated with the genetically determined kidney fibrosis progression rate in mice and the pathogenesis of FSGS in humans. This tubular C3 overproduction can, through local pro-fibrotic effects, influence the progression of chronic kidney disease.

Urinary complement biomarkers in immune-mediated kidney diseases

The complement system, an important part of the innate system, is known to play a central role in many immune mediated kidney diseases. All parts of the complement system including the classical, alternative, and mannose-binding lectin pathways have been implicated in complement-mediated kidney injury. Although complement components are thought to be mainly synthesized in the liver and activated in the circulation, emerging data suggest that complement is synthesized and activated inside the kidney leading to direct injury. Urinary complement biomarkers are likely a better reflection of inflammation within the kidneys as compared to traditional serum complement biomarkers which may be influenced by systemic inflammation. In addition, urinary complement biomarkers have the advantage of being non-invasive and easily accessible. With the rise of therapies targeting the complement pathways, there is a critical need to better understand the role of complement in kidney diseases and to develop reliable and non-invasive biomarkers to assess disease activity, predict treatment response and guide therapeutic interventions. In this review, we summarized the current knowledge on urinary complement biomarkers of kidney diseases due to immune complex deposition (lupus nephritis, primary membranous nephropathy, IgA nephropathy) and due to activation of the alternative pathway (C3 glomerulopathy, thrombotic microangiography, ANCA-associated vasculitis). We also address the limitations of current research and propose future directions for the discovery of urinary complement biomarkers.

Tubular toxicity of proteinuria and the progression of chronic kidney disease

Proteinuria is a well-established biomarker of chronic kidney disease (CKD) and a risk predictor of associated disease outcomes. Proteinuria is also a driver of CKD progression toward end-stage kidney disease. Toxic effects of filtered proteins on proximal tubular epithelial cells enhance tubular atrophy and interstitial fibrosis. The extent of protein toxicity and the underlying molecular mechanisms responsible for tubular injury during proteinuria remain unclear. Nevertheless, albumin elicits its toxic effects when degraded and reabsorbed by proximal tubular epithelial cells. Overall, healthy kidneys excrete over 1000 individual proteins, which may be potentially harmful to proximal tubular epithelial cells when filtered and/or reabsorbed in excess. Proteinuria can cause kidney damage, inflammation and fibrosis by increasing reactive oxygen species, autophagy dysfunction, lysosomal membrane permeabilization, endoplasmic reticulum stress and complement activation. Here we summarize toxic proteins reported in proteinuria and the current understanding of molecular mechanisms of toxicity of proteins on proximal tubular epithelial cells leading to CKD progression.

Pioglitazone Protects Tubular Epithelial Cells during Kidney Fibrosis by Attenuating miRNA Dysregulation and Autophagy Dysfunction Induced by TGF-β

Excessive renal TGF-β production and pro-fibrotic miRNAs are important drivers of kidney fibrosis that lack any efficient treatment. Dysfunctional autophagy might play an important role in the pathogenesis. We aimed to study the yet unknown effects of peroxisome proliferator-activated receptor-γ (PPARγ) agonist pioglitazone (Pio) on renal autophagy and miRNA dysregulation during fibrosis. Mouse primary tubular epithelial cells (PTEC) were isolated, pre-treated with 5 µM pioglitazone, and then stimulated with 10 ng/mL TGF-β1 for 24 h. Male 10-week-old C57Bl6 control (CTL) and TGF-β overexpressing mice were fed with regular chow (TGF) or Pio-containing chow (20 mg/kg/day) for 5 weeks (TGF + Pio). PTEC and kidneys were evaluated for mRNA and protein expression. In PTEC, pioglitazone attenuated (p < 0.05) the TGF-β-induced up-regulation of Col1a1 (1.4-fold), Tgfb1 (2.2-fold), Ctgf (1.5-fold), Egr2 (2.5-fold) mRNAs, miR-130a (1.6-fold), and miR-199a (1.5-fold), inhibited epithelial-to-mesenchymal transition, and rescued autophagy function. In TGF mice, pioglitazone greatly improved kidney fibrosis and related dysfunctional autophagy (increased LC3-II/I ratio and reduced SQSTM1 protein content (p < 0.05)). These were accompanied by 5-fold, 3-fold, 12-fold, and 2-fold suppression (p < 0.05) of renal Ccl2, Il6, C3, and Lgals3 mRNA expression, respectively. Our results implicate that pioglitazone counteracts multiple pro-fibrotic processes in the kidney, including autophagy dysfunction and miRNA dysregulation.

Aristolocholic acid I promotes renal tubular epithelial fibrosis by upregulating matrix metalloproteinase-9 expression via activating the C3a/C3aR axis of macrophages

Aristolochic acid I (AAI) can cause nephrotoxicity and is characterized by interstitial fibrosis. The C3a/C3aR axis of macrophages and matrix metalloproteinase-9 (MMP-9) play important roles in fibrosis, but whether they are involved in AAI-induced renal interstitial fibrosis and are related remains to be elucidated. In this study, we investigated whether C3a/C3aR axis of macrophages promotes renal interstitial fibrosis by regulating MMP-9 in aristolochic acid nephropathy (AAN). Intraperitoneal injection of AAI for 28 days successfully induced AAN in C57bl/6 mice. The content of C3a in the kidney of AAN mice was increased, and there was a significant distribution of macrophages in the renal tubules. The same results were observed in the in vitro experiment. We also explored the role and mechanism of macrophages after AAI administration in the epithelial-mesenchymal transformation (EMT) of renal tubular epithelial cells (RTECs) and found that AAI could activate the C3a/C3aR axis of macrophages to upregulate p65 expression in macrophages. p65 upregulated MMP-9 expression in macrophages not only directly but also by promoting the secretion if interleukin-6 by macrophages and then activating STAT3 in RTECs. The upregulation of MMP-9 expression could promote the EMT of RTECs. Taken together, our study demonstrated that the AAI-activated the C3a/C3aR axis of macrophages, which induced MMP-9 production, was one of the causes of renal interstitial fibrosis. Therefore, targeting the C3a/C3aR axis of macrophages is an effective therapeutic strategy for the prevention and treatment of renal interstitial fibrosis in AAN.

C3aR contributes to unilateral ureteral obstruction-induced renal interstitial fibrosis via the activation of the NLRP3 inflammasome

AIMS

Complement component 3a and its receptor (C3a/C3aR) and nucleotide-binding oligomerization domain-like receptor protein-3 (NLRP3) inflammasome are involved in the pathogenesis of renal interstitial fibrosis (RIF). However, the mechanisms have not been clearly illuminated. This study aimed to elucidate the roles of C3aR and the NLRP3 inflammasome involved in unilateral ureteral obstruction (UUO)-induced renal interstitial fibrosis.

MAIN METHODS

UUO models were established using male C57BL/6 wild-type (WT) mice and age-matched C3aR-deficient mice. MCC950, an inhibitor of the NLRP3 inflammasome, was intraperitoneally injected in UUO mice. Blood samples were collected to quantify serum creatinine and urea. Kidney samples were collected for hematoxylin-eosin (HE), Masson, and immunohistochemistry staining, terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay, and Western blotting.

KEY FINDINGS

Renal function, renal fibrosis, and renal inflammation in WT mice were aggravated with longer periods of UUO. C3aR deficiency improved renal function and attenuated renal fibrosis and the activation of the NLRP3 inflammasome in UUO mice. Renal function and renal fibrosis in UUO mice were attenuated after NLRP3 inflammasome inhibition; however, the expression of C3aR did not change.

SIGNIFICANCE

Our data revealed that C3aR may aggravate RIF by regulating the activation of the NLRP3 inflammasome (particularly regulating inflammasome assembly) in renal tubular epithelial cells in the UUO model.

Complement C3-targeted therapy in C3 glomerulopathy, a prototype of complement-mediated kidney diseases

C3 glomerulopathy (C3G) is a rare and complex kidney disease that primarily affects young adults. Renal outcomes remain poor in the absence of specific treatment. C3G is driven by uncontrolled overactivation of the alternative complement pathway, which is mainly of acquired origin. Functional characterization of complement abnormalities (i.e., autoantibodies targeting complement components and variants in complement genes) identified in patients and experimental models of the disease improved the understanding of the disease, making C3G a prototype of complement-mediated diseases. The contribution of C3 convertase, as well as C5 convertase, in disease occurrence, phenotype, and severity is now well established, offering various potential therapeutic interventions. However, the lack of sufficient efficiency in anti-C5 therapy highlights the extreme complexity of the disease and the need for new therapeutic approaches based on C3 and C3 convertase axis inhibition. Here, we provide an overview of the complement activation mechanism involved in C3G and discuss therapeutic options based on complement inhibitors, with a specific focus on C3 inhibition.

Urinary complement proteins and risk of end-stage renal disease: quantitative urinary proteomics in patients with type 2 diabetes and biopsy-proven diabetic nephropathy

PURPOSE

To investigate the association between urinary complement proteins and renal outcome in biopsy-proven diabetic nephropathy (DN).

METHODS

Untargeted proteomic and Kyoto Encyclopedia of Genes and Genomes (KEGG) functional analyses and targeted proteomic analysis using parallel reaction-monitoring (PRM)-mass spectrometry was performed to determine the abundance of urinary complement proteins in healthy controls, type 2 diabetes mellitus (T2DM) patients, and patients with T2DM and biopsy-proven DN. The abundance of each urinary complement protein was individually included in Cox proportional hazards models for predicting progression to end-stage renal disease (ESRD).

RESULTS

Untargeted proteomic and functional analysis using the KEGG showed that differentially expressed urinary proteins were primarily associated with the complement and coagulation cascades. Subsequent urinary complement proteins quantification using PRM showed that urinary abundances of C3, C9, and complement factor H (CFAH) correlated negatively with annual estimated glomerular filtration rate (eGFR) decline, while urinary abundances of C5, decay-accelerating factor (DAF), and CD59 correlated positively with annual rate of eGFR decline. Furthermore, higher urinary abundance of CFAH and lower urinary abundance of DAF were independently associated with greater risk of progression to ESRD. Urinary abundance of CFAH and DAF had a larger area under the curve (AUC) than that of eGFR, proteinuria, or any pathological parameter. Moreover, the model that included CFAH or DAF had a larger AUC than that with only clinical or pathological parameters.

CONCLUSION

Urinary abundance of complement proteins was significantly associated with ESRD in patients with T2DM and biopsy-proven DN, indicating that therapeutically targeting the complement pathway may alleviate progression of DN.

Understanding the Renal Fibrotic Process in Leptospirosis

Leptospirosis is a neglected infectious disease caused by pathogenic species of the genus Leptospira. The acute disease is well-described, and, although it resembles other tropical diseases, it can be diagnosed through the use of serological and molecular methods. While the chronic renal disease, carrier state, and kidney fibrosis due to Leptospira infection in humans have been the subject of discussion by researchers, the mechanisms involved in these processes are still overlooked, and relatively little is known about the establishment and maintenance of the chronic status underlying this infectious disease. In this review, we highlight recent findings regarding the cellular communication pathways involved in the renal fibrotic process, as well as the relationship between renal fibrosis due to leptospirosis and CKD/CKDu.

Role of intracellular complement activation in kidney fibrosis

Increased expression of complement C1r, C1s and C3 in kidney cells plays an important role in the pathogenesis of kidney fibrosis. Our studies suggest that activation of complement in kidney cells with increased generation of C3 and its fragments occurs by activation of classical and alternative pathways. Single nuclei RNA sequencing studies in kidney tissue from unilateral ureteral obstruction mice show that increased synthesis of complement C3 and C5 occurs primarily in renal tubular epithelial cells (proximal and distal), while increased expression of complement receptors C3ar1 and C5ar1 occurs in interstitial cells including immune cells like monocytes/macrophages suggesting compartmentalization of complement components during kidney injury. Although global deletion of C3 and macrophage ablation prevent inflammation and reduced kidney tissue scarring, the development of mice with cell-specific deletion of complement components and their regulators could bring further insights into the mechanisms by which intracellular complement activation leads to fibrosis and progressive kidney disease. LINKED ARTICLES: This article is part of a themed issue on Canonical and non-canonical functions of the complement system in health and disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.14/issuetoc.

Complement Components C3 and C4 Indicate Vasculitis Manifestations to Distinct Renal Compartments in ANCA-Associated Glomerulonephritis

Acute kidney injury (AKI) is a common and severe complication of antineutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV) causing progressive chronic kidney disease (CKD), end-stage renal disease (ESRD) or death. Pathogenic ANCAs, in particular proteinase 3 (PR3) and myeloperoxidase (MPO), trigger a deleterious immune response resulting in pauci-immune necrotizing and crescentic glomerulonephritis (GN), a common manifestation of glomerular injury in AAV. However, there is growing evidence that activation of the complement pathway contributes to the pathogenesis and progression of AAV. We here aimed to compare glomerular and tubulointerstitial lesions in ANCA GN and extrarenal manifestation of AAV in association with levels of circulating complement components C3c and C4.

METHODS

Plasma levels of C3c and C4 in a total number of 53 kidney biopsies with ANCA GN were retrospectively included between 2015 and 2020. Glomerular and tubulointerstitial lesions were evaluated according to established scoring systems for ANCA GN and analogous to the Banff classification.

RESULTS

We here show that circulating levels of C3c and C4 in ANCA GN were comparable to the majority of other renal pathologies. Furthermore, hypocomplementemia was only detectable in a minor subset of ANCA GN and not correlated with renal or extrarenal AAV manifestations. However, low levels of circulating C3c correlated with AKI severity in ANCA GN independent of systemic disease activity or extrarenal AAV manifestation. By systematic scoring of glomerular and tubulointerstitial lesions, we provide evidence that low levels of circulating C3c and C4 correlated with vasculitis manifestations to distinct renal compartments in ANCA GN.

CONCLUSIONS

We here expand our current knowledge about distinct complement components in association with vasculitis manifestations to different renal compartments in ANCA GN. While low levels of C4 correlated with glomerulitis, our observation that low levels of circulating complement component C3c is associated with interstitial vasculitis manifestation reflected by intimal arteritis implicates that C3c contributes to tubulointerstitial injury in ANCA GN.

The Complement C3a and C3a Receptor Pathway in Kidney Diseases

The pathogenesis of some kidney diseases is closely associated with complement activation, where the C3a/C3a receptor (C3aR) might play a crucial role. C3a/C3aR has dual roles and may exert anti-inflammatory or pro-inflammatory effects depending on different cell types and diseases. In the kidneys, C3aR is primarily expressed on the tubular epithelium and less in glomerular podocytes. C3aR expression is enhanced and the levels of C3a in the plasma and urine are increased in kidney diseases of several types, and are associated with disease progression and severity. The C3a/C3aR pathway facilitates the progression of glomerular and tubulointerstitial diseases, while it has opposite effects on urinary tract infections. Clinical trials targeting C3a/C3aR in kidney diseases are lacking. Here, we reviewed the studies on the C3a/C3aR pathway in kidney disease, with the aim of understanding in-depth its controversial roles and its potential therapeutic value.

Absence of the Lectin Activation Pathway of Complement Ameliorates Proteinuria-Induced Renal Injury

Proteinuria is an adverse prognostic feature in renal diseases. In proteinuric nephropathies, filtered proteins exert an injurious effect on the renal tubulointerstitium, resulting in inflammation and fibrosis. In the present study, we assessed to what extent complement activation via the lectin pathway may contribute to renal injury in response to proteinuria-related stress in proximal tubular cells. We used the well-established mouse model of protein overload proteinuria (POP) to assess the effect of lectin pathway inhibition on renal injury and fibrotic changes characteristic of proteinuric nephropathy. To this end, we compared experimental outcomes in wild type mice with MASP-2-deficient mice or wild type mice treated with MASP-2 inhibitor to block lectin pathway functional activity. Multiple markers of renal injury were assessed including renal function, proteinuria, macrophage infiltration, and cytokine release profiles. Both MASP-2-deficient and MASP-2 inhibitor-treated wild type mice exhibited renoprotection from proteinuria with significantly less tubulointerstitial injury when compared to isotype control antibody treated mice. This indicates that therapeutic targeting of MASP-2 in proteinuric nephropathies may offer a useful strategy in the clinical management of proteinuria associated pathologies in a variety of different underlying renal diseases.

Renal Inflammation and Fibrosis: A Double-edged Sword

Renal tissue injury initiates inflammatory and fibrotic processes that occur to promote regeneration and repair. After renal injury, damaged tissue releases cytokines and chemokines, which stimulate activation and infiltration of inflammatory cells to the kidney. Normal tissue repair processes occur simultaneously with activation of myofibroblasts, collagen deposition, and wound healing responses; however, prolonged activation of pro-inflammatory and pro-fibrotic cell types causes excess extracellular matrix deposition. This review focuses on the physiological and pathophysiological roles of specialized cell types, cytokines/chemokines, and growth factors, and their implications in recovery or exacerbation of acute kidney injury.

A Double-Edged Sword: Complement Component 3 in Toxoplasma gondii Infection

Sprague Dawley rats and Kunming (KM) mice are artificially infected with type II Toxoplasma gondii strain Prugniaud (Pru) to generate toxoplasmosis, which is a fatal disease mediated by T. gondii invasion of the central nervous system (CNS) by unknown mechanisms. The aim is to explore the mechanism of differential susceptibility of mice and rats to T. gondii infection. Therefore, a strategy of isobaric tags for relative and absolute quantitation (iTRAQ) is established to identify differentially expressed proteins (DEPs) in the rats' and the mice's brains compared to the healthy groups. In KM mice, which is susceptible to T. gondii infection, complement component 3 (C3) is upregulated and the tight junction (TJ) pathway shows a disorder. It is presumed that T. gondii-stimulated C3 disrupts the TJ of the blood-brain barrier in the CNS. This effect allows more T. gondii passing to the brain through the intercellular space.

Properdin binds independent of complement activation in an in vivo model of anti-glomerular basement membrane disease

Properdin is the only known positive regulator of complement activation by stabilizing the alternative pathway convertase through C3 binding, thus prolonging its half-life. Recent in vitro studies suggest that properdin may act as a specific pattern recognition molecule. To better understand the role of properdin in vivo, we used an experimental model of acute anti-glomerular basement membrane disease with wild-type, C3- and properdin knockout mice. The model exhibited severe proteinuria, acute neutrophil infiltration and activation, classical and alternative pathway activation, and progressive glomerular deposition of properdin, C3 and C9. Although the acute renal injury was likely due to acute neutrophil activation, we found properdin deposition in C3-knockout mice that was not associated with IgG. Thus, properdin may deposit in injured tissues in vivo independent of its main ligand C3.

Renal Tubular Complement 3 Deposition in Children with Primary Nephrotic Syndrome

Background

This study aimed to investigate the clinical significance of complement 3 (C3) deposition in renal tubules of children with primary nephrotic syndrome (PNS).

Methods

The clinical and pathological characteristics of PNS were retrospectively reviewed in 99 PNS pediatric patients, who were divided into the C3 deposition and the non-C3 deposition groups.

Results

A total of 39 patients (39.39%) had renal tubule C3 deposition. In the C3 deposition group, the ratios of urine N-acetylglucosaminidase/creatinine (UNAG/Cr), urine β2 microglobulin/creatinine (Uβ2MG/Cr), and urine transferrin/creatinine (UTRF/Cr) were significantly higher than those of the non-C3 deposition group. The patients of the C3 deposition group had lower serum total protein and albumin, higher cholesterol and D-dimer (DD), lower proportion of CD3+CD8+ cells, and higher proportion of CD19+CD23+ cells. The number of the patients with interstitial fibrosis, renal cell vacuolar degeneration, renal tubular immunoglobulin deposition, and severe tubulointerstitial injury in the C3 deposition group was higher than that of the non-C3 deposition group. The C3 deposition intensity was positively correlated with the number of recurrences.

Conclusion

PNS pediatric patients with C3 deposition in renal tubules have more severe disease condition, tubulointerstitial injury, and recurrence suggesting a worse long-term prognosis.

The C3aR promotes macrophage infiltration and regulates ANCA production but does not affect glomerular injury in experimental anti-myeloperoxidase glomerulonephritis

The anti-neutrophil cytoplasmic antibody (ANCA) associated vasculitides are autoimmune diseases associated with significant morbidity and mortality. They often affect the kidney causing rapidly progressive glomerulonephritis. While signalling by complement anaphylatoxin C5a though the C5a receptor is important in this disease, the role of the anaphylatoxin C3a signalling via the C3a receptor (C3aR) is not known. Using two different murine models of anti-myeloperoxidase (MPO) glomerulonephritis, one mediated by passive transfer of anti-MPO antibodies, the other by cell-mediated immunity, we found that the C3aR did not alter histological disease severity. However, it promoted macrophage recruitment to the inflamed glomerulus and inhibited the generation of MPO-ANCA whilst not influencing T cell autoimmunity. Thus, whilst the C3aR modulates some elements of disease pathogenesis, overall it is not critical in effector responses and glomerular injury caused by autoimmunity to MPO.

Many drugs for many targets: novel treatments for complement-mediated glomerular disease

There is a large body of experimental and clinical evidence that complement activation contributes to glomerular injury in multiple different diseases. However, the underlying mechanisms that trigger complement activation vary from disease to disease. Immune complexes activate the classical pathway of complement in many types of glomerulonephritis, whereas the alternative pathway and mannose-binding lectin pathways are directly activated in some diseases. Eculizumab is an inhibitory antibody to C5 that has been approved for the treatment of atypical hemolytic uremic syndrome, and case reports suggest that it is also effective in other types of glomerulonephritis. Furthermore, new complement-inhibitory drugs are being developed that target additional proteins within the complement cascade, raising the possibility of blocking the specific complement proteins involved in a given disease. This review examines the rationale for targeting different proteins within the complement cascade, the new anti-complement drugs currently in development and some of the challenges that investigators will face in bringing these drugs to the clinic.

Complement inhibition attenuates acute kidney injury after ischemia-reperfusion and limits progression to renal fibrosis in mice

The complement system is an essential component of innate immunity and plays a major role in the pathogenesis of ischemia-reperfusion injury (IRI). In this study, we investigated the impact of human C1-inhibitor (C1INH) on the early inflammatory response to IRI and the subsequent progression to fibrosis in mice. We evaluated structural damage, renal function, acute inflammatory response, progression to fibrosis and overall survival at 90-days post-injury. Animals receiving C1INH prior to reperfusion had a significant improvement in survival rate along with superior renal function when compared to vehicle (PBS) treated counterparts. Pre-treatment with C1INH also prevented acute IL-6, CXCL1 and MCP-1 up-regulation, C5a release, C3b deposition and infiltration by neutrophils and macrophages into renal tissue. This anti-inflammatory effect correlated with a significant reduction in the expression of markers of fibrosis alpha smooth muscle actin, desmin and picrosirius red at 30 and 90 days post-IRI and reduced renal levels of TGF-β1 when compared to untreated controls. Our findings indicate that intravenous delivery of C1INH prior to ischemic injury protects kidneys from inflammatory injury and subsequent progression to fibrosis. We conclude that early complement blockade in the context of IRI constitutes an effective strategy in the prevention of fibrosis after ischemic acute kidney injury.

Complement Component 3 Adapts the Cerebrospinal Fluid for Leptomeningeal Metastasis

We molecularly dissected leptomeningeal metastasis, or spread of cancer to the cerebrospinal fluid (CSF), which is a frequent and fatal condition mediated by unknown mechanisms. We selected lung and breast cancer cell lines for the ability to infiltrate and grow in CSF, a remarkably acellular, mitogen-poor metastasis microenvironment. Complement component 3 (C3) was upregulated in four leptomeningeal metastatic models and proved necessary for cancer growth within the leptomeningeal space. In human disease, cancer cells within the CSF produced C3 in correlation with clinical course. C3 expression in primary tumors was predictive of leptomeningeal relapse. Mechanistically, we found that cancer-cell-derived C3 activates the C3a receptor in the choroid plexus epithelium to disrupt the blood-CSF barrier. This effect allows plasma components, including amphiregulin, and other mitogens to enter the CSF and promote cancer cell growth. Pharmacologic interference with C3 signaling proved therapeutically beneficial in suppressing leptomeningeal metastasis in these preclinical models.

Targeting C3a/C5a receptors inhibits human mesangial cell proliferation and alleviates immunoglobulin A nephropathy in mice

Complement activation has a deep pathogenic influence in immunoglobulin (Ig)A nephropathy (IgAN). C3a and C5a, small cleavage fragments generated by complement activation, are key mediators of inflammation. The fragments exert broad proinflammatory effects by binding to specific receptors (C3aR and C5aR, respectively). However, no studies thus far have investigated the effects of C3a, C5a and their receptors on IgAN. We observed that C3aR and C5aR antagonists repressed IgA-induced cell proliferation and interleukin (IL)-6 and monocyte chemotactic protein 1 (MCP-1) production in cultured human mesangial cells (HMCs). Furthermore, an IgAN mouse model induced by Sendai virus infection was employed to investigate the effects of C3aR and C5aR on IgAN in vivo for the first time. Wild-type (WT) and several knock-out mouse strains (C3aR^-/- or C5aR^-/- ) were immunized intranasally with increasing doses of inactivated virus for 14 weeks and were subjected to two intravenous viral challenges during the time-period indicated. In the Sendai virus-induced IgAN model, C3aR/C5aR-deficient mice had significantly reduced proteinuria, lower renal IgA and C3 deposition, less histological damage and reduced mesangial proliferation compared with WT mice. Both C3aR deficiency and C5aR deficiency, especially C3aR deficiency, inhibited renal tumour necrosis factor (TNF)-α, transforming growth factor (TGF)-β, IL-1β, IL-6 and MCP-1 expression significantly. However, C3aR/C5aR-deficient and WT mice with IgAN did not differ with respect to their blood urea nitrogen (BUN) and serum creatinine levels. Our findings provide further support for the idea that C3aR and C5aR are crucially important in IgAN, and suggest that pharmaceutically targeting C3aR/C5aR may hold promise for the treatment of IgAN.

Pericytes and immune cells contribute to complement activation in tubulointerstitial fibrosis

We have examined the pathogenic role of increased complement expression and activation during kidney fibrosis. Here, we show that PDGFRβ-positive pericytes isolated from mice subjected to obstructive or folic acid injury secrete C1q. This was associated with increased production of proinflammatory cytokines, extracellular matrix components, collagens, and increased Wnt3a-mediated activation of Wnt/β-catenin signaling, which are hallmarks of myofibroblast activation. Real-time PCR, immunoblots, immunohistochemistry, and flow cytometry analysis performed in whole kidney tissue confirmed increased expression of C1q, C1r, and C1s as well as complement activation, which is measured as increased synthesis of C3 fragments predominantly in the interstitial compartment. Flow studies localized increased C1q expression to PDGFRβ-positive pericytes as well as to CD45-positive cells. Although deletion of C1qA did not prevent kidney fibrosis, global deletion of C3 reduced macrophage infiltration, reduced synthesis of C3 fragments, and reduced fibrosis. Clodronate mediated depletion of CD11bF4/80 high macrophages in UUO mice also reduced complement gene expression and reduced fibrosis. Our studies demonstrate local synthesis of complement by both PDGFRβ-positive pericytes and CD45-positive cells in kidney fibrosis. Inhibition of complement activation represents a novel therapeutic target to ameliorate fibrosis and progression of chronic kidney disease.

Relationship of clusterin with renal inflammation and fibrosis after the recovery phase of ischemia-reperfusion injury

Background

Long-term outcomes after acute kidney injury (AKI) include incremental loss of function and progression towards chronic kidney disease (CKD); however, the pathogenesis of AKI to CKD remains largely unknown. Clusterin (CLU) is a chaperone-like protein that reduces ischemia-reperfusion injury (IRI) and enhances tissue repair after IRI in the kidney. This study investigated the role of CLU in the transition of IRI to renal fibrosis.

Methods

IRI was induced in the left kidneys of wild type (WT) C57BL/6J (B6) versus CLU knockout (KO) B6 mice by clamping the renal pedicles for 28 min at the body temperature of 32 °C. Tissue damage was examined by histology, infiltrate phenotypes by flow cytometry analysis, and fibrosis-related gene expression by PCR array.

Results

Reduction of kidney weight was induced by IRI, but was not affected by CLU KO. Both WT and KO kidneys had similar function with minimal cellular infiltration and fibrosis at day 14 of reperfusion. After 30 days, KO kidneys had greater loss in function than WT, indicated by the higher levels of both serum creatinine and BUN in KO mice, and exhibited more cellular infiltration (CD8 cells and macrophages), more tubular damage and more severe tissue fibrosis (glomerulopathy, interstitial fibrosis and vascular fibrosis). PCR array showed the association of CLU deficiency with up-regulation of CCL12, Col3a1, MMP9 and TIMP1 and down-regulation of EGF in these kidneys.

Conclusion

Our data suggest that CLU deficiency worsens renal inflammation and tissue fibrosis after IRI in the kidney, which may be mediated through multiple pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/s12882-016-0348-x) contains supplementary material, which is available to authorized users.

The Immune System in Tissue Environments Regaining Homeostasis after Injury: Is "Inflammation" Always Inflammation?

Inflammation is a response to infections or tissue injuries. Inflammation was once defined by clinical signs, later by the presence of leukocytes, and nowadays by expression of "proinflammatory" cytokines and chemokines. But leukocytes and cytokines often have rather anti-inflammatory, proregenerative, and homeostatic effects. Is there a need to redefine "inflammation"? In this review, we discuss the functions of "inflammatory" mediators/regulators of the innate immune system that determine tissue environments to fulfill the need of the tissue while regaining homeostasis after injury.

Contribution of the anaphylatoxin receptors, C3aR and C5aR, to the pathogenesis of pulmonary fibrosis

Complement activation, an integral arm of innate immunity, may be the critical link to the pathogenesis of idiopathic pulmonary fibrosis (IPF). Whereas we have previously reported elevated anaphylatoxins-complement component 3a (C3a) and complement component 5a (C5a)-in IPF, which interact with TGF-β and augment epithelial injury in vitro, their role in IPF pathogenesis remains unclear. The objective of the current study is to determine the mechanistic role of the binding of C3a/C5a to their respective receptors (C3aR and C5aR) in the progression of lung fibrosis. In normal primary human fetal lung fibroblasts, C3a and C5a induces mesenchymal activation, matrix synthesis, and the expression of their respective receptors. We investigated the role of C3aR and C5aR in lung fibrosis by using bleomycin-injured mice with fibrotic lungs, elevated local C3a and C5a, and overexpression of their receptors via pharmacologic and RNA interference interventions. Histopathologic examination revealed an arrest in disease progression and attenuated lung collagen deposition (Masson's trichrome, hydroxyproline, collagen type I α 1 chain, and collagen type I α 2 chain). Pharmacologic or RNA interference-specific interventions suppressed complement activation (C3a and C5a) and soluble terminal complement complex formation (C5b-9) locally and active TGF-β1 systemically. C3aR/C5aR antagonists suppressed local mRNA expressions of tgfb2, tgfbr1/2, ltbp1/2, serpine1, tsp1, bmp1/4, pdgfbb, igf1, but restored the proteoglycan, dcn Clinically, compared with pathologically normal human subjects, patients with IPF presented local induction of C5aR, local and systemic induction of soluble C5b-9, and amplified expression of C3aR/C5aR in lesions. The blockade of C3aR and C5aR arrested the progression of fibrosis by attenuating local complement activation and TGF-β/bone morphologic protein signaling as well as restoring decorin, which suggests a promising therapeutic strategy for patients with IPF.-Gu, H., Fisher, A. J., Mickler, E. A., Duerson, F., III, Cummings, O. W., Peters-Golden, M., Twigg, H. L., III, Woodruff, T. M., Wilkes, D. S., Vittal, R. Contribution of the anaphylatoxin receptors, C3aR and C5aR, to the pathogenesis of pulmonary fibrosis.

Fine Mapping Implicates a Deletion of CFHR1 and CFHR3 in Protection from IgA Nephropathy in Han Chinese

An intronic variant at the complement factor H (CFH) gene on chromosome 1q32 (rs6677604) associates with risk of IgA nephropathy (IgAN), but the association signal has not been uniformly replicated in Han Chinese populations. We investigated whether the causal sequence variant resides in the CFH gene or the neighboring complement factor H-related 1 (CFHR1) gene and CFHR3, which harbor an 84-kb combined deletion (CFHR3,1Δ) in linkage disequilibrium with rs6677604. Imputation of 1000 Genomes Project data did not suggest new causal single-nucleotide variants within the CFH cluster. We next performed copy number analysis across the CFH locus in two independent Han Chinese case-control cohorts (combined n=3581). The CFHR3,1Δ and rs6677604-A alleles were rare (4.4% in patients and 7.1% in controls) and in strong linkage disequilibrium with each other (r²=0.95); of these alleles, CFHR3,1Δ associated more significantly with decreased risk of IgAN (odds ratio [OR], 0.56; 95% confidence interval [95% CI], 0.46 to 0.70; P=8.5 × 10^-8 versus OR, 0.61; 95% CI, 0.50 to 0.75; P=1.6 × 10^-6 for rs6677604-A). Moreover, CFHR3,1Δ explained all of the association signal at rs6677604 and remained significant after conditioning on rs6677604 genotype (P=0.01). Exploratory analyses of clinical and histopathologic parameters using the Oxford classification criteria revealed a suggestive association of CFHR3,1Δ with reduced tubulointerstitial injury (OR, 0.46; 95% CI, 0.25 to 0.79). These data indicate that dysregulated activity of the alternative complement pathway contributes to IgAN pathogenesis in both Asians and Europeans and implicate CFHR3,1Δ as the functional allele at this locus.

Complement involvement in kidney diseases: From physiopathology to therapeutical targeting

Complement cascade is involved in several renal diseases and in renal transplantation. The different components of the complement cascade might represent an optimal target for innovative therapies. In the first section of the paper the authors review the physiopathology of complement involvement in renal diseases and transplantation. In some cases this led to a reclassification of renal diseases moving from a histopathological to a physiopathological classification. The principal issues afforded are: renal diseases with complement over activation, renal diseases with complement dysregulation, progression of renal diseases and renal transplantation. In the second section the authors discuss the several complement components that could represent a therapeutic target. Even if only the anti C5 monoclonal antibody is on the market, many targets as C1, C3, C5a and C5aR are the object of national or international trials. In addition, many molecules proved to be effective in vitro or in preclinical trials and are waiting to move to human trials in the future.

C3a receptor antagonist ameliorates inflammatory and fibrotic signals in type 2 diabetic nephropathy by suppressing the activation of TGF-β/smad3 and IKBα pathway

OBJECTIVE

Diabetic nephropathy (DN) is a serious complication for patients with diabetes mellitus (DM). Emerging evidence suggests that complement C3a is involved in the progression of DN. The aim of this study was to investigate the effect of C3a Receptor Agonist (C3aRA) on DN and its potential mechanism of action in rats with type 2 diabetes mellitus (T2DM).

METHODS

T2DM was induced in SD rats by a high fat diet (HFD) plus repeated low dose streptozocin (STZ) injections. T2DM rats were treated with vehicle or C3aRA for 8 weeks. Biochemical analysis, HE and PAS stains were performed to evaluate the renal function and pathological changes. Human renal glomerular endothelial cells (HRGECs) were cultured and treated with normal glucose (NG), high glucose (HG), HG+C3a, HG+C3a+C3aRA and HG+C3a+BAY-11-7082 (p-IKBα Inhibitor) or SIS3 (Smad3 Inhibitor), respectively. Real-time PCR, immunofluorescent staining and western blot were performed to detect the mRNA and protein levels, respectively.

RESULTS

T2DM rats showed worse renal morphology and impaired renal function compared with control rats, including elevated levels of serum creatinine (CREA), blood urea nitrogen (BUN) and urine albumin excretion (UACR), as well as increased levels of C3a, C3aR, IL-6, p-IKBα, collagen I, TGF-β and p-Smad3 in the kidney of T2DM rats and C3a-treated HRGECs. In contrast, C3aRA treatment improved renal function and morphology, reduced CREA, UACR and the intensity of PAS and collagen I staining in the kidney of T2DM rats, and decreased C3a, p-IKBα, IL-6, TGF-β, p-Smad3 and collagen I expressions in HRGECs and T2DM rats.

CONCLUSION

C3a mediated pro-inflammatory and pro-fibrotic responses and aggravated renal injury in T2DM rats. C3aRA ameliorated T2DN by inhibiting IKBα phosphorylation and cytokine release, and also TGF-β/Smad3 signaling and ECM deposition. Therefore, complement C3a receptor is a potential therapeutic target for DN.

The role of complement in the pathogenesis of renal ischemia-reperfusion injury and fibrosis

The complement system is a major component of innate immunity and has been commonly identified as a central element in host defense, clearance of immune complexes, and tissue homeostasis. After ischemia-reperfusion injury (IRI), the complement system is activated by endogenous ligands that trigger proteolytic cleavage of complement components via the classical, lectin and/or alternative pathway. The result is the formation of terminal complement components C3a, C5a, and the membrane attack complex (C5b-9 or MAC), all of which play pivotal roles in the amplification of the inflammatory response, chemotaxis, neutrophil/monocyte recruitment and activation, and direct tubular cell injury. However, recent evidence suggests that complement activity transcends innate host defense and there is increasing data suggesting complement as a regulator in processes such as allo-immunity, stem cell differentiation, tissue repair, and progression to fibrosis. In this review, we discuss recent advances addressing the role of complement as a regulator of IRI and renal fibrosis after organ donation for transplantation. We will also briefly discuss currently approved therapies that target complement activity in kidney ischemia-reperfusion and transplantation.

The role of complement in the pathogenesis of renal ischemia-reperfusion injury and fibrosis

The complement system is a major component of innate immunity and has been commonly identified as a central element in host defense, clearance of immune complexes, and tissue homeostasis. After ischemia-reperfusion injury (IRI), the complement system is activated by endogenous ligands that trigger proteolytic cleavage of complement components via the classical, lectin and/or alternative pathway. The result is the formation of terminal complement components C3a, C5a, and the membrane attack complex (C5b-9 or MAC), all of which play pivotal roles in the amplification of the inflammatory response, chemotaxis, neutrophil/monocyte recruitment and activation, and direct tubular cell injury. However, recent evidence suggests that complement activity transcends innate host defense and there is increasing data suggesting complement as a regulator in processes such as allo-immunity, stem cell differentiation, tissue repair, and progression to fibrosis. In this review, we discuss recent advances addressing the role of complement as a regulator of IRI and renal fibrosis after organ donation for transplantation. We will also briefly discuss currently approved therapies that target complement activity in kidney ischemia-reperfusion and transplantation.

Complement regulation in renal disease models

Activation of the complement system is tightly regulated by plasma and cell-associated complement regulatory proteins (CRPs), such as factor H (fH), decay-accelerating factor, and membrane cofactor protein. Animal models of disease have provided considerable insights into the important roles for CRPs in the kidney. Mice deficient in fH have excessive fluid phase C3 activation and inactivation, leading to deposition of inactivated C3b in glomerular capillary walls (GCW), comparable with dense deposit disease. In contrast, when fH lacks C-terminal surface targeting regions, local activation on the GCW leads to a disease reminiscent of thrombotic microangiopathy. The uniquely rodent protein, CR1-related y (Crry), has features analogous to human membrane cofactor protein. Defective Crry leads to unrestricted alternative pathway activation in the tubulointerstitium, resulting in pathologic features ranging from thrombotic microangiopathy (TMA), acute kidney injury, and tubulointerstitium nephritis. In the presence of initiators of the classic or lectin pathways, commonly in the form of immune complexes in human glomerular diseases, complement regulation is stressed, with the potential for recruitment of the spontaneously active alternative pathway. The threshold for this activation is set by CRPs; pathology is more likely when complement regulation is defective. Within the endocapillary region of the GCW, fH is key, while decay-accelerating factor and Crry are protective on mesangial cells and podocytes. Arguably, acquired alterations in these CRPs is a more common event, extending from pathologic states of cellular injury or production of inhibitory antibodies, to physiological fine tuning of the adaptive immune response.

Loss of CD11b exacerbates murine complement-mediated tubulointerstitial nephritis

Acute complement activation occurs in the tubulointerstitium (TI) of kidneys transplanted from Crry^−/−C3^−/− mice into complement-sufficient wildtype mice, followed by marked inflammatory cell infiltration, tubular damage and interstitial fibrosis. We postulated iC3b-CD11b interactions were critical in this TI nephritis model. We transplanted Crry^−/−C3^−/− mouse kidneys into CD11b^−/− and wildtype C57BL/6 mice. Surprisingly, there was greater inflammation in Crry^−/−C3^−/− kidneys in CD11b^−/− recipients compared to those in wildtype hosts. Kidneys in CD11b^−/− recipients had large numbers of CD11b⁻Ly6C^hiCCR2^hiF4/80⁺ cells consistent with inflammatory (M1) macrophages recruited from circulating monocytes of the host CD11b^−/− animal. There was also an expanded population of CD11b⁺CD11c⁺Ly6C⁻F4/80^hi cells. Since these cells were CD11b⁺, they must have originated from the transplanted kidney; their surface protein expression and appearance within the kidney were consistent with the intrinsic renal mononuclear cellular population. These cells were markedly expanded relative to all relevant controls, including the contralateral donor kidney and Crry^−/−C3^−/− mouse kidneys in CD11b^+/+ wildtype recipients. Direct evidence for their in situ proliferation was the presence of nuclear Ki67 and PCNA in CD11b⁺F4/80⁺ cells. Thus, in this experimental model in which there is unrestricted C3 activation, CD11b⁺ monocytes limit their own infiltration into the kidney and prevent proliferation of endogenous mononuclear cells. This suggests a role for outside-in iC3b-CD11b signals in limiting intrinsic organ inflammation.

LMW heparin prevents increased kidney expression of proinflammatory mediators in (NZBxNZW)F1 mice

We have previously demonstrated that continuous infusion of low molecular weight (LMW) heparin delays autoantibody production and development of lupus nephritis in (NZBxNZW)F1 (B/W) mice. In this study we investigated the effect of LMW heparin on renal cytokine and chemokine expression and on nucleosome-mediated activation of nucleosome-specific splenocytes. Total mRNA extracted from kidneys of heparin-treated or -untreated B/W mice was analysed by qPCR for the expression of several cytokines, chemokines, and Toll-like receptors. Splenocytes taken from B/W mice were stimulated with nucleosomes with or without the presence of heparin. Splenocyte cell proliferation as thymidine incorporation and the expression of costimulatory molecules and cell activation markers were measured. Heparin treatment of B/W mice reduced the in vivo expression of CCR2, IL1 β , and TLR7 compared to untreated B/W mice. Nucleosome-induced cell proliferation of splenocytes was not influenced by heparin. The expression of CD80, CD86, CD69, CD25, CTLA-4, and TLR 2, 7, 8, and 9 was upregulated upon stimulation by nucleosomes, irrespective of whether heparin was added to the cell culture or not. In conclusion, treatment with heparin lowers the kidney expression of proinflammatory mediators in B/W mice but does not affect nucleosomal activation of splenocytes.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

Novel roles of complement in renal diseases and their therapeutic consequences

The complement system functions as a part of the innate immune system. Inappropriate activation of the complement pathways has a deleterious effect on kidneys. Recent advances in complement research have provided new insights into the pathogenesis of glomerular and tubulointerstitial injury associated with complement activation. A new disease entity termed 'C3 glomerulopathy' has recently been proposed and is characterized by isolated C3 deposition in glomeruli without positive staining for immunoglobulins. Genetic and functional studies have demonstrated that several different mutations and disease variants, as well as the generation of autoantibodies, are potentially associated with its pathogenesis. The data from comprehensive analyses suggest that complement dysregulation can also be associated with hemolytic uremic syndrome and more common glomerular diseases, such as IgA nephropathy and diabetic kidney disease. In addition, animal studies utilizing genetically modified mice have begun to elucidate the molecular pathomechanisms associated with the complement system. From a diagnostic point of view, a noninvasive, MRI-based method for detecting C3 has recently been developed to serve as a novel tool for diagnosing complement-mediated kidney diseases. While novel therapeutic tools related to complement regulation are emerging, studies evaluating the precise roles of the complement system in kidney diseases will still be useful for developing new therapeutic approaches.

Autoantibody-mediated complement C3a receptor activation contributes to the pathogenesis of preeclampsia

Preeclampsia (PE) is a prevalent life-threatening hypertensive disorder of pregnancy associated with increased complement activation. However, the causative factors and pathogenic role of increased complement activation in PE are largely unidentified. Here we report that a circulating maternal autoantibody, the angiotensin II type 1 receptor agonistic autoantibody, which emerged recently as a potential pathogenic contributor to PE, stimulates deposition of complement C3 in placentas and kidneys of pregnant mice via angiotensin II type 1 receptor activation. Next, we provide in vivo evidence that selectively interfering with C3a signaling by a complement C3a receptor-specific antagonist significantly reduces hypertension from 167±7 to 143±5 mm Hg and proteinuria from 223.5±7.5 to 78.8±14.0 μg of albumin per milligram creatinine (both P<0.05) in angiotensin II type 1 receptor agonistic autoantibody-injected pregnant mice. In addition, we demonstrated that complement C3a receptor antagonist significantly inhibited autoantibody-induced circulating soluble fms-like tyrosine kinase 1, a known antiangiogenic protein associated with PE, and reduced small placental size with impaired angiogenesis and intrauterine growth restriction. Similarly, in humans, we demonstrate that C3 deposition is significantly elevated in the placentas of preeclamptic patients compared with normotensive controls. Lastly, we show that complement C3a receptor activation is a key mechanism underlying autoantibody-induced soluble fms-like tyrosine kinase 1 secretion and decreased angiogenesis in cultured human villous explants. Overall, we provide mouse and human evidence that angiotensin II type 1 receptor agonistic autoantibody-mediated activation contributes to elevated C3 and that complement C3a receptor signaling is a key mechanism underlying the pathogenesis of the disease. These studies are the first to link angiotensin II type 1 receptor agonistic autoantibody with complement activation and to provide important new opportunities for therapeutic intervention in PE.

The C5a receptor has a key role in immune complex glomerulonephritis in complement factor H-deficient mice

Chronic serum sickness leads to the formation of glomerular immune complexes; however, C57BL/6 mice do not develop glomerulonephritis unless complement factor H (CFH) is absent from the plasma. Here we studied the role for C5a receptor (R) in this setting. The exaggerated humoral immune response in CFH^−/− mice was normalized in CFH^−/−C5aR^−/− double knockout mice, highlighting the C5aR dependence. The CFH knockout mice developed proliferative glomerulonephritis with endocapillary F4/80⁺ macrophage infiltration, a process reduced in the double knockout mice. There was no interstitial inflammation by histologic criteria or flow cytometry for F4/80⁺Ly6C^hiCCR2^hi inflammatory macrophages. There were, however, more interstitial CD3⁺CD4⁺ T lymphocytes in CFH knockout mice with chronic serum sickness, while double knockout mice had greater than 5-fold more Ly6C^loCCR2^lo anti-inflammatory macrophages compared to the CFH knockout mice. Mice lacking C5aR were significantly protected from functional renal disease as assessed by blood urea nitrogen levels. Thus, IgG- and iC3b-containing immune complexes are not inflammatory in C57BL/6 mice. Yet when these mice lack CFH, sufficient C3b persists in glomeruli to generate C5a and activate C5aR.